The role of exogenous iron in the activation of ribonucleotide reductase from Escherichia coli

Protein R2, the small component of ribonucleotide reductase from Escherichia coli, contains a diferric center and a catalytically essential tyrosyl radical. In vitro, this radical can be produced in the protein diiron center and lacking the tyrosyl radical, and from two inactive forms, metR2, containing an intact apoR2, lacking both iron and the radical. While activation of apoR2 requires only a source of ferrous iron and exposure to O-2, activation of metR2 was achieved using a multienzymatic system consisting of an NAD(P)H:flavin oxidoreductase, superoxide dismutase and a poorly defined protein fraction, named fraction b (Fontecave M, Eliasson R, Reichard P (1987) J Biol Chem 262:12325-12331). In both reactions, reduced R2, containing a diferrous center, is a key intermediate which is subsequently converted to active R2 during reaction with O-2. By in vivo labeling of E. coli with radioactive Fe-59, we show that fraction b contains iron. Depletion of the iron in fraction b inactivates it, and fraction b can be substituted for by ferric citrate solutions. Furthermore, aqueous Fe2+ in the presence of dithiothreitol is able to convert metR2 into reduced R2. Therefore we propose that the function of fraction b is to provide, in association with the flavin reductase, ferrous iron for reduction of the endogenous diiron center. Since fraction b is not a single well-defined protein, it remains to be shown whether, in vivo, that function resides in a specific protein. Exogenous iron can thus participate in activation of both apoR2 and metR2, but it is incorporated into R2 only in the former case. A unifying mechanism is proposed.